[processor/deltatorate] Correctly transform unit of rates

[mx-psi]

Description

Since we are calculating a rate per second, we want the unit to be transformed to "original units" per second.
This PR appends /s to the unit of generated rates if the unit is set.

I argue this is compliant with UCUM (the unit spec used by OpenTelemetry) no matter what the original unit is, because (quoting the UCUM spec):

The semantics of The Unified Code for Units of Measure implies equivalence classes such that different expressions may have the same meaning.

and

Terms are evaluated from left to right with the period and the solidus having the same operator precedence. Multiple division operators are allowed within one term.

Therefore units like m/s/s or ms/s are valid UCUM units.

Since the unit may be missing, to avoid assuming the unit is "1" we don't add /s in that case.